Go to the stage builder, move any existing custom stages on Brawl to your SD card, and exit the stage builder. Smash Stack will not work if the console has any custom stages on it, even the three samples that come with a new file or the stages from the Smash Service.

Connect the SD card to your computer and rename the "private" folder to "privateold" in the root of the SD card.

Now copy/extract the files in this ZIP archive and the data ZIP to the root of the SD card.